From: goldenfieldquaternions@gmail.com   
      
   On Monday, February 5, 2018 at 11:30:39 AM UTC-6, questions...@gmail.com wrote:   
   > I understand that light from stars is coherent and can be treated   
   > as a plane wave. I wonder how can I calculate the amplitude A of   
   > such plane wave A exp[ct-kx] for a given star form its magnitude,   
   > bandwidth, distance and other parameters of the star.   
   >   
   > What is the typical range for A for a typical wavelength?   
   >   
   > Thanks   
   >   
   > [[Mod. note --   
   > 1. The light from a star is *incoherent* -- each of the huge number   
   >    of atoms in the star's photosphere is radiating independently, and   
   >    the light we receive is (that tiny fraction that happens to be   
   >    radiated in our direction) the incoherent sum of light from many   
   >    of those atoms.   
   > 2. The light from a star is coming from very far away, so to a *very*   
   >    good approximation it can be treated as a plane wave.   
   > 3. The amplitude of a plane wave is directly related to the intensity   
   >    of the light, i.e., how bright the star is.  The book "Astrophysical   
   >    Quantities", by Allen, has numbers for home many photons/second   
   >    per square centimeter of detector area we receive for a given   
   >    magnitude star, but I don't recall these offhand.  Converting to   
   >    an amplitude of a plane wave takes a little bit more algebra...   
   > -- jt]]   
      
   Coherent light a plane wave does not make. There is a funny issue   
   in quantum optics that one can't make perfectly coherent light.   
   There is always some element of Gaussian noise. Conversely one can't   
   make completely incoherent light either. The Hanbury Brown and Twiss   
   illustrates how light from a thermal source can enter into entanglements   
   at the measurement device. It is a form of the Wheeler Delayed   
   Choice Experiment. Because of this, and more fundamentally from the   
   Einstein coefficients there is no such thing as a perfectly coherent   
   or incoherent source of radiation.   
      
   LC   
      
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